This invention relates to distributed fiber optic sensor arrays such as are employed in shipboard hydrophone systems for sensing changes in physical phenomena and more particularly to the provision of a system employing distributed arrays of optically switched fiber optic sensor arrays.
A conventional fiber optic hydrophone array that is immersed in water and pulled behind a ship for sensing acoustic vibrations in the water generally comprises a plurality of spaced apart (i.e., distributed) fiber optic sensors connected in series or in a ladder configuration or in a matrix configuration. In practice, several hydrophone arrays are connected in series behind the vessel with a number of input and output fiber optic buses being required and extending back to processing equipment on the towing vessel for each array or subarray. Such fiber optic sensor arrays are well known in the art, being described in publications and patents such as: U.S. Pat. No. 4,632,551, issued Dec. 30, 1986 for Passive Sampling Interferometric Sensor Arrays by G. A. Pavlath; U.S. Pat. No. 4,697,926, issued Oct. 6, 1987 for Coherent Distributed Sensor and Method Using Short Coherence Length Sources by R. C. Youngquist, etal; U.S. Pat. No. 4,699,513, issued Oct. 13, 1987 for Distributed Sensor and Method Using Coherence Multiplexing of Fiber-Optic Interferometeric Sensors by Janet L. Brooks, etal; U.S. Pat. No. 4,770,535, issued Sep. 13, 1988 for Distributed Sensor Array and Method Using a Pulse Signal Source by B. Y. Kim, etal; U.S. Pat. No. 4,789,240, issued Dec. 6, 1988 for Wavelength Switched Passive Interferometric Sensor System by I. J. Bush; U.S. Pat. No. 4,818,064, issued Apr. 4, 1989 for Sensor Array and Method of Selective Interferometric Sensing by Use of Coherent Synthesis by R. C. Youngquist, etal; U.S. Pat. No. 4,889,986, issued Dec. 26, 1989 for Serial Interferometric Fiber-Optic Sensor Array by A. D. Kersey, etal; U.S. Pat. No. 5,011,262, issued Apr. 30, 1991 for Fiber Optic Sensor Array by M. R. Layton; and U.S. Pat. No. 5,039,221, issued Aug. 13, 1991 for Interferometer Calibration for Fiber Optic Sensor Arrays by M. R. Layton, etal, which are incorporated herein by reference.
In a fiber optic ladder sensor array, for example, a different sensor is connected in each rung of a ladder structure having input and output fiber optic buses connected to opposite ends of each sensor. The input bus is connected to receive light from a single light source. Each input light pulse on the input bus is sequentially applied to each sensor through an associated directional coupler. The result is a diminution of the input light pulse at each sensor as the pulse progresses along the input bus of the array, the maximum number of distributed sensors of the array being determined by the minimum amount of light that can be passed on the output bus and detected by electronic equipment on the towing vessel. It is desirable to be able to increase the number of sensors that can be towed behind a vessel without increasing the number of optical buses required in the tow cable and without increasing the amount of processing equipment required on the towing vessel.
An object of this invention is the provision of an improved optical fiber hyrdrophone sensor array system. Another object is the provision of an optical fiber sensor array that requires fewer optical fiber buses than conventional arrays. Another object is the provision of an optical fiber sensor array or subarrays in which the optical input signal that is passed to distributed subarrays is not reduced in intensity as it passes the location of each subarray. Another object is the provision of an optical fiber sensor array in which separate input light pulses of substantially the same magnitude are applied to each subarray of the array. A further object is to increase the number of sensors driven by a single source.